Fiber-optic sensors are increasingly being used as devices for sensing some quantity, typically temperature or mechanical strain, but sometimes also displacements, vibrations, pressure, acceleration, rotations, or concentrations of chemical species. The general principle of such devices is that light from a laser is sent through an optical fiber and there experiences subtle changes of its parameters either in the fiber itself or in one or several point-location sensing fiber Bragg gratings and then reaches a detector arrangement which measures these changes.
In particular a growing application field is the use of fiber optic sensing system for acoustic sensing, especially Distributed Acoustic Sensing (DAS). DAS optical fibers can be deployed into almost any region of interest and used to monitor for occurrences that generate acoustic perturbations. DAS is quickly becoming recognized as a powerful tool for remote sensing in oil and gas operations. The list of existing and potential applications in remote sensing for this new technology continues to grow and includes not only downhole or subsurface applications but other applications in which acoustic perturbations are of interest, such as subsea umbilical's and risers, and in the security field for perimeter security. Basically any structure can be monitored for acoustic perturbations in this way. Traditionally, DAS applications in the subsurface environment use pulsed electromagnetic waves to interrogate a fiber optic cable for sensing acoustic and vibration phenomena in an oil well, or reservoir. This type of sensor is sometimes referred to as a time-domain coherent optical reflectometer and utilizes a technique called time division multiplexing. In summary, a short electromagnetic coherent pulse (usually in the infrared) is injected into one end of a fiber optic. Pulses are back reflected or backscattered via Rayleigh scattering along a continuum of virtual reflectors in the fiber and these pulses are analyzed using interferometric techniques. A phase of the returned light is measured that is related to the local stretch in the fiber optic during its exposure to an acoustic pressure wave. The optical phase ideally will vary linearly with the acoustic pressure wave. Once a light pulse is injected, a period of time should be surpassed before injecting another pulse of light. This amount of time is twice the transit time of light from the injection location to the end of the fiber. This is done to ensure there is no light in the fiber when another pulse of light is injected. The pulse repetition frequency of the DAS is the reciprocal of the wait time between light injections. Half of the pulse repetition frequency is the well-known Nyquist frequency, which is the maximum acoustic bandwidth available for monitoring.
As the business intensity grows in the worldwide campaign to find and produce more oil there is increasing need to better monitor subsurface oil field operations using more sophisticated acoustic monitoring. In particular there are increasingly applications in which there is a need for detecting much higher frequency and higher bandwidth acoustic signals than that available with time division multiplexing alone. Examples include an increasing interest in listening for sand flow, high bandwidth telemetry, listening for proppant in hydraulic fracturing operations, measuring fluid flow by acoustic signatures (particularly with active ultrasonic flow monitoring systems), monitoring flow regimes, listening for wellbore leaks (often high frequency), listening for cavitation in flow, listening for plug leaks or inter-zone leaks, monitoring vortex shedding, and wireline sonic logging. These applications require a sensitive listening device with an increased audio bandwidth and an improved signal-to-noise ratio.
The technical approach to be described in this application does not rely on the pulsed laser time division multiplexing described above.